Problem: Evaluate $8p +3q-18$ when $p=\dfrac12$ and $q=7$.
Let's substitute $ p={ \dfrac12}$ and $ q=7$ into the expression. $\phantom{=}8 {p} +3{q}-18$ $=8\left({{\dfrac12}}\right)+3({7})-18$ $=4+21-18$ $=7$